The present invention relates to an optical disk apparatus for recording and/or reproducing information in and/or from an optical disk.
As one of the causes of a servo follow-up deviation in the optical disk apparatus, a periodic disturbance is included therein, which is caused by rotating the optical disk. There has been a method that makes a servo gain high to increase an allowable disturbance amount, as one of the methods for enhancing a follow-up performance against the periodic disturbance.
In contrast, JP-A-2001-84620 has disclosed that “It is necessary to raise a cutoff frequency since a servo band is more broadened when a reproduction speed of DVD is made high. For example, an oscillation is inevitable when a resonance appears by setting the cutoff frequency to 10 kHz.”
As another method of enhancing the follow-up performance against the periodic disturbance, the iterative learning control has been known beforehand.
JP-A-11-213403 has discloses a technique that detects a vibration equal to or greater than a predetermined value in which the vibration is added externally to an optical disk apparatus, discontinues an operation that increases a gain of a rotational frequency range in the optical disk in accordance with the detected signal, and increases an entire servo band of a servo control.
JP-A-8-77589 has disclosed a technique that varies a learning level of a iterative learning control, and turns down the learning level thereof when a track correlation is weak.
JP-A-2001-126421 has discloses the iterative learning control that provides a configuration including an adder that adds an output signal of a filter to a position error signal (PES), which is not delayed, to output to a delay memory device as a new inner state signal.
On the other hand, it is known that there is a component so called deviation, other than an eccentricity/surface run-out component as the periodic disturbance component occurred at a time of rotating the optical disk. U.S. patent application Publication No. 2007/0183284 has disclosed that “In the case of optical disk having a guide groove, a defective portion (deviation) in a groove shape is sometimes included by causing a deterioration of disk stamper, a disk forming defect, etc. A wide band inherent noise component is mixed in a groove reflected signal at an outer circumference portion of the disk in particular, when the above-mentioned disk is rotated in high operation speed.”
Further, JP-A-2003-67951 has disclosed a method that calculates a defect zone, decreases a loop gain of the servo control in the calculated defect zone to turn down the cutoff frequency, and turns up the loop gain in other than the defect zone to make the cutoff frequency high.